What is something that almost nobody knows about retiring early?

To most, retirement means able to live a life wo having to worry about money and many see it as a life goal whether young or old. The thing is, we should retire with a sustainable intention and not just for the sake of not having to work anymore/sleep in/holidays etc. All these may seem perfect but no matter how good something is, a constant repetitive routine will make things boring. Like what elon musk tweeted after he became the richest man in the world, " well, back to work". Lucky for investors, imagine if he said, "screw this, im out of Tesla." If i'm gonna retire at my 40s, what am I gonna do for the next 20 years? Hopefully there will be an answer to it.​​​

I think we need to consider that our retirement expenses may not be the same as our current expenses. (And hence our projections of how much we need in retirement may be under-estimates.)

Maybe we're working part-time in retirement and dedicate some time to leisure, travel, holiday. Maybe we're meeting up with friends/family more regularly, spending more on get-togethers. Etc. Can't really say like, oh at 25 years old today I need $2k monthly so that's what I'll need in retirement. Then need to account for inflation also - $2k per month at 25 will not be the same amount at 45 years old.

I know for a fact that I intend to spend way more in retirement than I do now. So while I'm spending within my means now, I'm also planning and investing to achieve a much larger monthly expenditure in retirement.​​​